The present invention relates to a method of manufacturing a nitride semiconductor substrate for use in a visible light emitting diode or a blue violet laser.
Group III-V nitride semiconductor such as gallium nitride (GaN), indium nitride (InN) and aluminum nitride (AlN) is preferably used as a compound semiconductor material for a blue or green light emitting diode (LED), a blue semiconductor laser or a high speed transistor device capable of operating at a high temperature. There is a well-known substrate to grow nitride semiconductor thereon such as an insulating substrate of sapphire (monocrystalline Al2O3), silicon carbide (SiC), silicon (Si) or gallium arsenic (GaAs).
However, it is known that if nitride semiconductor is grown on a substrate of a different material such as sapphire, the difference between the thermal expansion coefficients of the nitride semiconductor to grow and the substrate causes the substrate to bow or have cracks, which degrades the crystallinity of the nitride semiconductor.
In recent years, there have been attempts to solve the problem related to the difference between the materials of the substrate and the layer grown thereon by forming the substrate with nitride semiconductor and forming an element structure of the same kind of nitride semiconductor thereon.
According to one method of manufacturing a nitride semiconductor substrate, for example, a nitride semiconductor layer is grown to have a relatively large thickness on a substrate to be a base member (base member substrate), and a laser beam is irradiated on the interface between the grown nitride semiconductor layer and the base substrate. According to the proposed method, the nitride semiconductor layer irradiated with the laser beam is locally heated and sublimed, and separated from the base substrate, so that a nitride semiconductor substrate may be provided from the nitride semiconductor layer.
According to the conventional method of manufacturing the nitride semiconductor substrate, when the nitride semiconductor layer is separated from the base substrate, however, only the part of the interface being irradiated with the laser beam between the nitride semiconductor layer and the base substrate is separated, while the other part remains connected. In this case, stress concentrates on the connected part of the nitride semiconductor layer and the base substrate, and cracks are generated in the nitride semiconductor layer. This makes it difficult to manufacture the nitride semiconductor substrate with high yield by irradiation of a laser beam about at a room temperature.
At the time of growing nitride semiconductor on a base substrate, threading defects caused by lattice mismatch are introduced, so that a resulting nitride semiconductor substrate has a high defect density.
The present invention is directed to a solution to the above disadvantage associated with the conventional method and it is an object of the present invention to surely provide a highly productive nitride semiconductor substrate free from cracks and having a reduced defect density.
In order to achieve the above object, according to the present invention, a mask film is formed. The mask film is used to selectively grow a semiconductor layer of nitride on a main surface of a base substrate.
More specifically, a first method of manufacturing a nitride semiconductor substrate according to the present invention includes a first step of forming a mask film of a material on which substantially no nitride semiconductor grows and having a plurality of openings on a main surface of a base substrate, a second step of selectively growing a semiconductor layer of nitride on the base substrate through the mask film, and a third step of irradiating an interface between the semiconductor layer and the base substrate with a laser beam, thereby separating the semiconductor layer from the base substrate to form a semiconductor substrate from the semiconductor layer.
According to the first manufacturing method, the semiconductor layer is selectively grown on the base substrate through the mask film, and therefore stress can be concentrated on the mask film, so that the stress generated in the semiconductor layer can be reduced. As a result, breaks or cracks generated in the semiconductor layer can be reduced. In addition, since a material on which substantially no semiconductor layer grows is used for the mask film, the semiconductor layer grows over the mask film. Therefore, threading defects introduced into the semiconductor layer can be reduced. Thus, a nitride semiconductor substrate having high crystal quality and allowing high productivity can be provided.
Preferably in the first method, the base substrate is composed of sapphire whose main surface is in a {0001} plane orientation, and in the first step, each opening is formed in a stripe shape substantially in a direction of a zone axis, a  less than 1-100 greater than  direction in the base substrate. In this manner, with respect to the sapphire whose main surface is the {0001} plane forming the base substrate, the zone axis direction of the semiconductor of the nitride grown thereon is shifted by 30xc2x0. Therefore, the stripe shaped opening in the mask film is formed to have its lengthwise direction arranged along the zone axis direction of the base substrate, the  less than 1-100 greater than  direction, so that the surface of the semiconductor layer growing to extend over the mask film can be formed into a good {1-101} plane.
Preferably in the first method, the base substrate is composed of silicon carbide or aluminum nitride whose main surface is in a {0001} plane orientation, and in the first step, each opening is formed in a stripe shape in a direction of the zone axis, a  less than 11-20 greater than  direction in the base substrate. Thus, the zone axis of the silicon carbide or aluminum nitride forming the base substrate whose main surface is the {0001} plane and the zone axis of the semiconductor layer of nitride grown thereon are in coincidence. As a result, when the stripe shaped opening in the mask film is formed to have its lengthwise direction arranged along the zone axis of the base substrate, the  less than 11-20 greater than  direction, the growing surface of the semiconductor layer to extend over the mask film can be a good {1-101} plane.
Preferably, the first method further includes the step of forming an irregular region on the main surface of the base substrate before the first step, and the first step includes the step of forming the mask film so that a top surface of a raised portion in the irregular region is exposed through the opening.
Thus, when a semiconductor layer is formed on the base substrate through the mask film, stress is concentrated on a raised part in the irregular region formed on the main surface of the base substrate, and therefore the stress caused in the growing semiconductor layer is reduced. As a result, breaks or cracks in the semiconductor layer during the growth are more reduced.
Preferably in this case, the base substrate is composed of sapphire whose main surface is in a {0001} plane orientation, and the step of forming the irregular region includes the step of forming a plurality of grooves extending parallel to each other on the main surface of the base substrate so that the grooves are substantially in a direction of a zone axis, a  less than 1-100 greater than  direction in the base substrate.
Also preferably in this case, the base substrate is composed of silicon carbide or aluminum nitride whose main surface is in a {0001} plane orientation, and the step of forming the irregular region includes the step of forming a plurality of grooves parallel to each other on the main surface of the base substrate so that the grooves are substantially in a direction of a zone axis, a  less than 11-20 greater than  direction in the base substrate.
Also preferably in this case, the first step includes the steps of forming a mask forming film on the entire surface of the irregular region in the base substrate, applying a resist film to cover the mask forming film, etching the resist film while leaving the resist film on the recessed part of the irregular region, thereby exposing an upper part of the raised part of the irregular region in the mask forming film, and etching the mask forming film using the resist film left on the recessed part as a mask.
Further in this case, oxygen plasma is preferably used in the step of etching the resist film.
In this case, the mask forming film is preferably composed of an oxide.
In the second step of forming the irregular region on the main surface of the base substrate, a gap is preferably formed between the base substrate and the semiconductor layer. In this way, the gap allows heat generated at the time of laser beam irradiation to concentrate on the interface between the semiconductor layer and the base substrate, which improves the thermal efficiency. As a result, a high output light source is not necessary for the laser irradiating system, which can consequently contribute to a reduction in the manufacturing cost. In addition, since a high-pressure nitrogen gas from the semiconductor layer generated by thermal decomposition during the laser beam irradiation can effectively be diffused, so that the possibility of cracks being introduced into the semiconductor layer at the time of separation can be more reduced.
Preferably, in the first method, in the third step, a laser beam is irradiated upon at least a part of the semiconductor layer exposed through an opening in the mask film.
In the first method, in the first step, the plurality of openings are preferably formed in an island shape, and in the third step, a laser beam is preferably irradiated while scanning in synchronization with a part of the semiconductor layer exposed through each opening in the mask film. In this way, a pulsed laser source can be used as a light source for the laser beam, and therefore the output value of the laser beam can be increased, which reduces the time for the laser beam irradiation. In addition, the base substrate and the semiconductor layer can surely be separated.
Also preferably in the first method, in the first step, the plurality of openings are formed in an island shape, and in the third step, a laser beam is irradiated upon a plurality of exposed parts of the semiconductor layer at a time while scanning the exposed parts through openings in the mask film. In this way, a plurality of exposed parts of the semiconductor layer through the openings in the mask film can be irradiated at a time, the time for laser beam irradiation can be more reduced.
Preferably in the first method, the mask film is composed of at least one selected from the group consisting of silicon oxide, silicon nitride, and tungsten.
Preferably in the first method, an interval of the ends of adjacent openings in the mask film is substantially equal to or smaller than a thickness of the semiconductor layer.
Preferably in the first method, a width of the openings in the mask film is at most about ten times as large as an interval of the ends of adjacent openings.
A second method of manufacturing a nitride semiconductor substrate includes a first step of selectively etching a main surface of a base substrate and forming an irregular region on the main surface of the base substrate, a second step of growing a semiconductor layer of nitride on the irregular region in the base substrate so that a gap is formed between the layer and a recess in the irregular region and the upper surface is flat, and a third step of irradiating a laser beam upon an interface between the semiconductor layer and the base substrate to separate the semiconductor layer from the base substrate, thereby forming a semiconductor substrate from the semiconductor layer.
According to the second method, when the semiconductor layer grows, stress can be concentrated on the raised portions of the irregular region of the base substrate, and therefore the stress caused in the semiconductor layer can be reduced. As a result, breaks or cracks caused in the semiconductor layer during the growth can be prevented. Furthermore, the semiconductor layer grows to extend over the recessed portions in the base substrate, and therefore threading defects introduced in the semiconductor layer can be reduced. As a result, a nitride semiconductor substrate having high quality and allowing high productivity can be provided. Meanwhile, the manufacturing process can be simplified because the mask film is not necessary on the base substrate.
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